1. Field of the Invention
The present invention relates to a developing device for developing an electrostatic image on an image carrying member, used in an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus.
2. Related Background Art
Conventionally proposed as a configuration of apparatus for forming a multi-color image by the electrophotographic method is a method for locating a plurality of developing devices containing respective developing agents or developers of different colors (each of which will be referred to as "toner") on a rotating/selecting mechanism with respect to a photosensitive drum being an electrophotographic, photosensitive member, opposing either one developing device containing the toner of a predetermined color to the photosensitive drum to form a developed image, transferring this developed image onto a recording medium, and further carrying out these developing and transferring operations for each of the colors, thereby obtaining the multi-color image. Some apparatuses employ the cartridge structure for the foregoing developing devices arranged to be detachably mounted on the image forming apparatus, thereby lightening the maintenance work of user.
Such developing devices are normally constructed in such a way that a toner supply roll obtained by attaching a sponge roll to a metal core is pressed against a developing roller in order to supply the toner in a toner container to the developing roller being a toner carrying member.
In such toner supply structure, air pressure inside the toner container increases with rotation of the developing roller and toner supply roll, and the air therein leaks from a portion of weak seal strength to the outside.
In order to prevent the toner inside the toner container from leaking out with the air at this time, a pressure relief portion for relief of the air within the toner container to the outside is provided and a filter is attached to the pressure escape portion, thereby allowing only the air to escape to the outside.
With use of the conventional developing devices the filter, however, tends to cause clogging due to the toner or the like and the filter might be damaged when pressure is applied on the filter because of movement of toner or the like in the toner container during physical distribution or when something hits the toner container from the outside.